This invention relates to an image formation apparatus of a so-called digital electrophotographic apparatus for uniformly charging a photosensitive body on which an electrostatic latent image is formed, irradiating the charged photosensitive body with a light beam modulated in response to image information, thereby forming an electrostatic latent image on the photosensitive body, and developing the electrostatic latent image to form a visible image.
In recent years, the image formation apparatus of a so-called digital electrophotographic apparatus for irradiating a photosensitive body with a light beam supporting image information representing characters, pictures, etc., thereby forming an electrostatic latent image on the photosensitive body as described above has been widely adopted for printers and copiers.
In such a digital image formation apparatus, hitherto an apparatus for turning on/off a light beam at high speed to form not only a monochrome binary image, but also a gray-scale image and forming an electrostatic latent image of a so-called halftone dot structure or full line structure, thereby representing halftone has been known. This apparatus, which uses a comparatively easy algorithm and can be realized at low cost, has been widely adopted for printers and copiers of the digital electrophotographic apparatus.
In such an apparatus for forming an electrostatic latent image of a so-called halftone dot structure or full line structure, thereby representing halftone, a latent image is formed in a constant light beam spot diameter and a constant number of dots or lines per unit length from the low-density portion (highlight portion) to the high-density portion in an image. Thus, the exposure profile in the highlight portion becomes an exposure profile having an intermediate (analog) exposure energy distribution lowering in contrast wherein exposure energy does not change binarily (digitally) as on/off, and further the exposure amount itself is also small. Thus, an image poor in reproducibility of dots and full lines and low in granularity is provided; stability of gradation representation against temperature and humidity environments is also poor. To reproduce small characters, particularly, small kanji characters, line reproducibility is also poor and crush and thin spots occur.
To overcome the problem, if the number of dots or lines per unit length is decreased for forming a latent image in order to improve the exposure profile contrast in the highlight portion, the reproducibility of dots and full lines in the highlight portion can be enhanced, but the image structure of dots, etc., becomes easy to recognize and resolution of characters also lowers, leading to low image quality.
An apparatus for making the light beam spot diameter sufficiently small is also possible, in which case an image formation optical channel for collecting a light beam for forming a light beam spot on a photosensitive body becomes too precise and expensive and is too upsized to be practical.
On the other hand, to overcome the problem, disclosed in the Unexamined Japanese Patent Application Publication No. Hei. 1-169454 is means for enhancing exposure profile contrast by using a photosensitive body having a light potential attenuation characteristic in which the charge potential is not attenuated until a reasonable exposure energy strength, and is attenuated abruptly when the exposure energy strength exceeds one threshold value. Also, the Japanese Patent Application Publication No. Hei. 2-282277 describes halftone image reproduction using a photosensitive body indicating a similar characteristic to that in the Unexamined Japanese Patent Application Publication No. Hei. 1-169454.
According to the method, the charge potential is not attenuated at a weak exposure energy strength, thus the weak exposure energy portion on the base of exposure profile is cut off and clear characters can be reproduced. Also, reproducibility of dots and full lines is made good and a rough image low in granularity is improved; an image excellent in granularity can be provided.
However, at the weak exposure energy strength, the charge potential changes abruptly when a dead area in which the charge potential is not attenuated is exceeded. Thus, gradation representation containing a highlight portion becomes steep and if an unevenness of light beam irradiation energy, etc., exists, easily becomes manifest as a density unevenness. To suppress the steep gradation representation and manifesting the density unevenness, it is still necessary to make the light beam spot diameter sufficiently small. As described above, the image formation optical channel becomes too precise and expensive and is too upsized to be practical.